Hey Owen,
I recently got my Lm GT1 18X10 shaved back for some gutter rash which was inherited when i bought the car... The Magman will prob charge u a large amount for these as the inside of the wheels need to separated from the othside of the rim as they need to reanodise the wheel. You can do separate this youself but it does take some time might be cheaper too.. just ask the place where u werre gone get it done at. The cost would be mainly due to the outside rim being aniodised. This is a surface coating which you will notice gives the outer part i.e the rim of the wheel a "shinner" look. When geting your wheels shaved back for gutter rash you loose this anionised effect.. in addition when they weld onto the rim to fill in deep gashs the aniodised finish may sometimes discolour... (purplish metal colour) so you should be aware of this before getting them welded.
Any way i reckon they are an awesome rims and pretty rare, so fixing them might be a good idea...you need to do the balance thou, if they gonna charge you too much then its prob not worth it.
Hope some of this information helps
Cheers